How can we license Microsoft Power BI Report Server on-premises?
There are two possible ways to license Power BI Report Server on-premises.
- With a Power BI Premium subscription per capacity There are five plans available providing a certain amount of cores (so called “Capacity”) which can be used to execute the server. The purchased capacity can simultaneously be used as a cloud service as well as on premises.

The capacities for the most common plans are: P1 with 8 cores, P2 with 16 cores, and P3 with 32 cores.

2. Via Software Assurance benefits for SQL Server Enterprise
If your SQL Server Enterprise licenses are under active Software Assurance, you may run Power BI Report Server free of cost on the licensed servers or in Azure. Bare in mind the number of licensed cores. With virtualized instances, SQL Server Enterprise and Power BI Report Server do not have to run in the same OSE, but the underlying physical (and licensed) hardware is relevant and must be taken into account.

Sources:
https://powerbi.microsoft.com/de-de/pricing/
https://docs.microsoft.com/en-us/power-bi/admin/service-premium-what-is